Engineering question

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Dsg319 said:
Grounded system for example. Live conductor has failed and touching unbonded “floating”metallic appliance.

Human comes by barefoot on garage concrete floor and touches metallic appliance.

Current now flows through human through concrete/earth back to the grounded source.

Magnitude and severity of shock and current flow depending on many things such as, weather conditions (wet/dry), resistance of the human under those particular circumstances.
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Ok your example above lets say the conductor is protected by fuse right on the line side where it touches the unboned metallic enclosure.

Would the fault current move thru the line to fuse to the unbonded metallic enclosure to human back to source? Would Not the fuse trip?
 
hhsting said:
Ok your example above lets say the conductor is protected by fuse right on the line side where it touches the unboned metallic enclosure.

Would the fault current move thru the line to fuse to the unbonded metallic enclosure to human back to source? Would Not the fuse trip?
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If the enclosure is left with no EGC. And isolated from ground/earth, to less complicate things. There would be no current flow until the human comes in contact with both grounded floor,concrete, earth, and appliance and in theory completes the circuit.
And magnitude of current flow will likely not be enough to open any OCPD in most conditions.
 
Dsg319 said:
If the enclosure is left with no EGC. And isolated from ground/earth, to less complicate things. There would be no current flow until the human comes in contact with both grounded floor,concrete, earth, and appliance and in theory completes the circuit.
And magnitude of current flow will likely not be enough to open any OCPD in most conditions.
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And if the enclosure has EGC bonded? Current flow would be?
 
This is why we have GFCI's. The resistance of a person could be 10K ohms. On a 120V circuit that has faulted to an unbonded metal enclosure, current flow will be 120/10000 = 0.012 amps or 12mA when a person touches it if they are standing on the earth. That's enough to shock you, but way insufficient to trip a breaker. This probably won't kill a person, bit you'll feel it. If you are in bare feet, or sweaty, your skin resistance and resistance to earth could be well less than 10K ohms which will allow more current to flow which could kill you. It takes well under an amp to kill you, and fuses/breaker usually start at 15A for branch circuits. GFCI's for personnel protection must trip at 6mA of current leakage.

Current flow will be close to zero if a person isn't touching it. There could be some leakage to earth via the structure it is mounted on. But even a metal post in the ground is probably going to have a resistance of 50 ohms or more which will only allow a few amps to flow. That won't trip a breaker either.

If there is an EGC to the chassis, a common limit in UL standards is 100 mOhms (0.1 ohms) for the resistance of the EGC to the chassis. So in a 120V system with a bonding path resistance of 0.1 ohms plus the resistance of the EGC and ungrounded conductors in the circuit, the total resistance could be maybe 0.4 or 0.5 ohms in a long circuit. At 120V, this allows 120/0.5 = 240 amps to flow which would trip a 20A breaker via its magnetic trip mechanism.
 
hhsting said:
Neutral can function as both EGC and neutral? Whats wrong with that
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It can and does, but it is only permitted to do so on the line side of the service equipment.

In your application, with the neutral bonded at the outside disconnect, an open service neutral will result in a hazardous voltage on the disconnect enclosure if you have a neutral to enclosure bond. There is no connection to the rest of the electrical bonding system that may reduce the hazard by limiting the voltage to other bonded equipment. No matter what, an open service neutral may create a hazardous voltage to the earth itself.
 
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